The present invention relates to susceptors, and more particularly to susceptors useful in a chemical vapor deposition process occuring in a radiant absorption heater system.
Chemical vapor deposition processes are currently used to produce a variety of high purity materials and are of special utility in the production of III-V materials (such as gallium arsenide) and other epitaxial compositions used for semiconductors, etc.
The chemical vapor deposition process is typically performed in any of a variety of reactors well recognized in the art, including horizontal reactors, vertical reactors, pancake reactors and the like. Broadly, the reactors fall into two different classifications: radiant absorption heater systems (such as cold wall reactors) and oven reactors (such as hot wall reactors). Cold wall or radiant energy absorption systems are typically preferred over hot wall reactors for many reasons including their better yields and throughputs.
In the oven reactor, all of the equipment associated with the process is heated more-or-less to the reaction temperature of the source gases, which then deposit the material to be formed not only on the intended substrate, but also, for example, on the walls of the reactor vessel, thereby wasting both energy and the source materials, creating cleaning problems and, when different source materials are to be used for successive runs, presenting contamination problems between runs. By way of contrast, in a radiant absorption heating system, radiant energy in the form of radio frequency (RF), infra-red (IR) or microwave energy is projected from outside the reactor vessel into the reactor vessel where it is selectively absorbed by a susceptor. As the energy is absorbed by the susceptor, the susceptor becomes hotter than the surrounding elements of the reactor vessel and consequently selectively heats up a susceptor-carried substrate for the material to be deposited. While the selective heating of the substrate relative to the other reactor vessel elements is a clear advantage of the radiant absorption heater system, economic operation of such a system clearly requires a susceptor which will efficiently absorb the energy being introduced. The present invention is directed to a susceptor for use in such a radiant absorption heater system.
Various considerations severely restrict the selection of material for use as a susceptor. Clearly the material must be capable of efficiently absorbing the specific form of radiant energy being introduced into the reaction vessel so as to minimize the energy input required to bring the system to, and maintain it at, its usual operating temperature (this typically being in the neighborhood of 700.degree. C. or higher for the production of epitaxial III-V materials). Obviously the material must also be chemically inert with respect to the materials it contacts, including the source gases being introduced into the reaction vessel, and must not be a source of contamination to either the source gases, the substrate or the material being produced. When heated to the temperatures typically encountered by susceptors (often in excess of 1,000.degree. C.), most materials outgas--that is, they release from within gases which act as a source of contamination for the source gases, the substrates, and the material to be deposited, thereby introducing elements foreign to the desired reaction and resulting in an inferior end-product. Finally, use of the material must be economically feasible in the quantities required for susceptor use.
To illustrate the difficulties encountered in selecting an appropriate material for a susceptor, one has only to consider the difficulty encountered with materials typically suggested for such purposes. High purity quartz is not only relatively inert, but exhibits very little outgassing. On the other hand, high purity quartz is not a good absorber of radiant energy and therefore is not useful as a susceptor, although it has been used as a substrate holder in hot oven reactors. One can, of course, increase the level of absorption of radiant energy by high purity quartz simply by coating the quartz with black (for example, carbon black), but this introduces a major source of contamination defeating the purpose of using the high purity quartz in the first place.
Graphite (one form of carbon) and glassy carbon (another form of carbon) are both good absorbers of radiant energy, but both exhibit substantial outgassing at elevated temperatures. Furthermore, diodes fabricated with susceptors made of such materials have been found to possess "leaky" electrical characteristics. While these materials may be coated with silicon carbide to effect a seal of the graphite or glassy carbon and prevent outgassing therefrom, the silicon carbide is not itself inert to the source gases typically utilized in III--V epitaxy.
Silicon carbide by itself is difficult to formulate in the desired thickness for a susceptor, is too expensive for general use, and is reactive with certain source gases. Molybdenum is also unsatisfactory for various reasons.
Accordingly, it is an object of the present invention to provide a susceptor for use in a chemical vapor deposition process in a radiant absorption heater system which efficiently absorbs the radiant energy being employed, is economically feasible, is chemically inert with respect to the possible products and reactants, and is a minimal source of contaminants.
A further object is to provide such a susceptor which combines the high heat absorption of graphite, the low outgassing characteristic of silicon carbide, and the chemical inertness of high purity quartz to process gases and materials.